Sphenisciformes (Ksepka & Clarke, 2010)

Park, Travis & Fitzgerald, Erich M. G., 2012, A review of Australian fossil penguins (Aves: Sphenisciformes), Memoirs of Museum Victoria 69, pp. 309-325 : 309

The Sphenisciformes

Sphenisciformes (penguins) are a group of flightless marine birds confined to the southern hemisphere. Contrary to popular stereotype, not all species reside in Antarctica, with the highest species diversity found in New Zealand (Ksepka et al., 2012) and one species (the Galapagos penguin, Spheniscus mendiculus Sundevall, 1871 actually living at the equator ( Vargas et al., 2005; Jadwiszczak, 2009). Fossil species are found in the same regions as extant species ( Simpson, 1975), with Antarctica, Australia, New Zealand, South Africa, and South America all possessing both fossil and extant assemblages. Present regional species diversities roughly correspond to past levels, with areas such as New Zealand and Antarctica well represented by numerous fossil and living species, despite the fossil record not being continuous throughout the Cenozoic ( Ksepka & Ando, 2011; Ksepka et al., 2012)( Fig. 1 ). One of the most specialised avian groups ( Kaiser, 2007), the morphology of living penguins is well known ( Pycraft, 1898; Lowe, 1933; Marples, 1952) and they have evolved a range of adaptations to an aquatic lifestyle including: small and scalelike feathers; increased underwater visual acuity ( Sivak & Millodot, 1977; Bowmaker & Martin, 1985); several retia mirabilia systems for efficient thermoregulation ( Frost et al., 1975; Thomas & Fordyce, 2007, 2012); stiffening of wing joints (Raikow et al., 1998); relative shortening of the wing; hydrodynamic flattening of wing elements; and shortening of the tarsometatarsus. The Sphenisciformes differ from most avian groups in that total diversity was greater in the past than present, with 19 extant species (sensu Ksepka & Ando, 2011), and 53 recognised fossil species ( Fig. 2 ). This is testament to their aquatic lifestyle and the fact that particular penguin bones (e.g. humerus, tarsometatarsus) are more likely to fossilise than their equivalents in other avian groups due to their pachyostotic histology ( Meister, 1962). Extant species feed on small fish, cephalopods, crustaceans and plankton, and show little interspecific postcranial morphological variation ( Olson, 1985). Interspecific differences in cranial morphology are minimal; the differences that do occur probably reflect disparate feeding ecology ( Zusi, 1975).

Figures

Figure 1. Chronostratigraphy and correlation of Australian fossil penguin-bearing units, compared with the fossil record of Sphenisciformes on other continents. Note that the vertical ranges of stratigraphic units represent estimates of geologic age maxima and minima for penguin-bearing horizons, not necessarily discrete time-spans of deposition. Geologic timescale is after Gradstein et al. (2004) with updates from Walker and Geissman (2009). See Table 1 for references to ages of Australian units. Ages of African units: Hendey (1981); Roberts et al. (2011); Ksepka and Thomas (2012). Ages of Antarctic units: Dingle and Lavelle (2000); Jadwiszczak (2006); Marenssi et al. (2012). Ages of New Zealand units: White and Waterhouse (1993); Cooper (2004); Slack et al. (2006); Ksepka et al. (2012). Ages of South American units: Devries (1998); Scasso et al. (2001); Celma and Cantalamessa (2007); Achurra et al. (2009); Malumián and Náñez (2011); Uhen et al. (2011).

Figure 2. Temporally-calibrated phylogeny of Sphenisciformes modified from Ksepka and Ando (2011) and Ksepka et al. (2012), showing major trends in Sphenisciform evolution and the first occurrence of Sphenisciformes in each region with extant species. Stem sphenisciform branches are shown in black and crown clade Spheniscidae branches are shown in blue. Thick horizontal bars on branches indicate stratigraphic range maxima and minima. Abbreviations: AF, Africa; AN, Antarctica; AU, Australia; Ma, million years ago; SA, South America.